The invention relates to a process for the preparation of a composite crystalline aluminium silicate. More particularly it relates to a hydrothermal process for preparing same.
Crystalline aluminium silicates, which can be distinguished from one another by certain characteristic lines occurring in their X-ray powder diffraction patterns, can be prepared hydrothermally by maintaining an aqueous alkaline starting mixture comprising one or more silicon compounds, one or more aluminium compounds, one or more alkali metal compounds and one or more organic nitrogen compounds at an elevated temperature until the crystalline aluminium silicate has formed and subsequently separating the crystalline silicate from the mother liquor, usually followed by washing and drying.
The right selection of reaction components comprised in the aqueous mixture and their molar ratios is of major importance in preparing the desired crystalline aluminium silicates.
It is known that the microscopic texture (e.g. the crystal shape, size, degree of smoothness and the degree of agglomeration of the ultimate crystals) of a synthetic zeolite is influenced by the organic nitrogen compound(s) present in the starting mixture used to carry out the above-described hydrothermal process.
When producing larger amounts of composite crystalline aluminium silicate on a (semi-)commercial scale it is highly desirable that silicates of constant quality and composition are obtained. This implies that process mixtures which comprise a considerable number of compounds are preferably kept in motion continuously or intermittently (e.g. by stirring) in order to keep the composition of the mixtures reasonably constant over their whole volume.